Breathing gas mixtures of helium and oxygen are normally used in deep sea diving. As the speed of sound is much higher in helium than in air, the frequency of resonance of the diver's voice is increased while the periodicity or pitch of the voice is essentially unchanged. As the pressure increases with depth, the frequency of the resonance of the diver's voice is also increased such that at depths greater than about 300 feet, the effect of the helium-oxygen breathing gas and the pressure increases the resonance frequency of the voice to about twice normal. This results in nearly unintelligible speech. Greater depths further increase the resonance frequency of the voice such that at about 1500 feet, the resonant frequency is about 3.5 times normal and results in completely unintelligible speech from the diver.
Thus, it is advantageous to have a deep sea diving speech converter that converts the speech made in the helium-oxygen breathing gas at the diving depths to a normal resonant frequency of voice in air at the surface so that understandable communications can be made between the surface and the diver.